DESCRIPTION: We propose to develop a tunable resonator/cavity for a 25 W, 395 GHz gyrotron with an ultra-wideband tuning of more than 6 GHz (1.5%) with near-constant power across the band. The gyrotron will be used for Dynamic Nuclear Polarization (DNP) enhanced solid-state and solution-state NMR spectroscopy. With DNP, the inherently small signal intensities in an NMR experiment can be enhanced by several orders of magnitude. This significantly increased overall sensitivity will be highly beneficial for analytical applications o NMR spectroscopy as well as the structure determination of bio-macromolecules. Currently available gyrotrons, either commercial or under development in research labs, have a limited tuning range of < 0.1% with at least a factor of 10 roll-off in power at the higher frequencies. Hence, systems with such gyrotrons need NMR magnets with expensive sweep coils to allow DNP studies with different polarizing agents because at a fixed NMR field, the optimal frequency for highest enhancement can span well over 1% bandwidth. A gyrotron with an ultra-wide band tuning capability has several advantages, namely, it will (1) eliminate the expensive NMR sweep coil and the risk associated with sweeping the field over wide ranges, (2) enable a standard frequency gyrotron for upgrading currently deployed NMR magnets (without sweep coils and slightly different field values) in the field with a DNP upgrade without the need for recharging them and (3) allow investigation of novel polarizing agents. This novel tunable resonator/cavity will not involve any of the traditional tuning techniques such as mechanically movable parts or cavity deformation which, typically limit the lifetime of the gyrotron to a few tens of thousands o hours of operation and allow only a few hundred tuning cycles. Also, this novel cavity does not require additional warm-bore space in the gyrotron magnet and is compatible with current DNP gyrotron architecture. Hence, this novel concept ensures that the tuning bandwidth can be increased by a factor >15 without a negligible increase in system cost. In Phase I, we will design, build and cold test the novel gyrotron cavity to prove this advanced tuning concept. We will also complete the design of a tunable 395 GHz gyrotron with at least 25 W of power over 6 GHz of tuning bandwidth for DNP at 600 MHz. The successful verification of the innovative tuning concept with the cold test in Phase I will enable the successful development of a prototype gyrotron in Phase II. As an ultimate result of this project, we expect Bridge12 to deliver ultra-broad band tunable gyrotrons with >1.5% tuning centered around terahertz frequencies corresponding to NMR spectrometers in the 400 MHz (263 GHz) to 900 MHz (593 GHz) for DNP-NMR spectroscopy. This will greatly accelerate structure determination of bio- macromolecules of relevance to human disease research by NMR techniques.